S10 - Accretion disks

Question 1

What is viscosity and its signficance on mass and angular mometum transfer inside an accretion disk?

Answer 1

Viscosity can be thought of as a frictional force due to collisions between gas particles.
Viscosity in the accretion disk causes mass transfer inwards and angular momentum transfer outwards.
As time proceeds, the ring spreads and distributes more and more mass to smaller radii. More mass is transferred inwards than spread outwards causing the disk to accrete mass.

Question 2

How are disks around stars observed in space?

Answer 2

(1) Excessive infrared emission suggests presence of dust grains (dust absorbing light from star and re-emitting it at infrared wavelengths).
(2) Molecular line emission (e.g. hydrogen (main constituent of disk))
(3) Reflected/scattered light
(4) In silhouette against bright nebular background

Question 3

What are the challenges with observing disks?

Answer 3

Disks are typically about 100au in radius (typical star-forming regions are 100pc), so the protoplanetary disks only span a few arcsec on the sky. Outer, cold regions of protoplanetary disks only emit at long wavelengths (beyond FIR). Interferometry is needed to resolve sub-mm thermal emission from protoplanetary disks.

Question 4

What causes rings in disks that surround young stars?

Answer 4

The rings are formed by planets sweeping up material in the disk.

Question 5

How can the presence of dusty disks around young stars be inferred indirectly?

Answer 5

Via measurement of the spectral energy distributions (SEDs) of young stars.

Question 6

Describe the evolution of infrared spectral energy distributions of a star as it develops.

Answer 6

In the early stages of formation, the dusty envelope is very optically thick, even at IR wavelengths: spectrum peaks in far-IR. As the envelope clears, the peak shifts to shorter wavelengths until light from the star and disk is revealed. Eventually the disk is dispersed, leaving behind emission from the star only.
As young stars evolve, the peak in the SED shifts to shorter wavelengths, as the evelope and then the disk gradually disappear (as most accretion and outflow ends), and the star is gradually revelead.